fuel trim what?
 

i plugged in my obd2 plug. as expected my one oxygen sensor (left bank) is not functioning correctly. its not so bad as to activate check engine light but it does not cycle properly like the other one.

however on the obd2 app it shows my car having 2 fuel trims. the one cycles correctly but the other one just flatlines at like 80 or something and does not move or spike or anything. im a bit clueless when it comes to this stuff. is the fuel trim a sensor that can be replaced? the car does have poor economy recently. or will replacing the o2 sensor fix it? why 2 fuel trims?

Fuel trim is a measure of how much the amount of fuel is being messed with, compared to the basic amount that is supposed to get injected. You have two of them most likely because you have two banks of cylinders. The one is stuck at 80 almost certainly because the O2 sensor on that bank is dead. Fix it, and the trim reading should start behaving much better.

Note that there are short-term trims and long-term trims as well. The long-term one doesn't tend to change much, while the short-term one changes much more. I would suggest that your second trim is the long-term one, but a value of 80 doesn't sound like a LFT value.

-soD

On most obd2 cars, there are two measurements of fuel trim: short-term and long-term. Short term fuel trims(SFT) are the immediate readings that the ECU sees from the O2 sensor. These readings will fluctuate rapidly in a cycle pattern, usually around 0 if the O2 is working well.

Long term fuel trims I'm unsure about, but I think they are what the ECU sees as the direction that it is "bent" towards. Having a high positive number denotes that your car is running lean and will try to richen the mixture as you drive. Negative numbers is the opposite.

What vehicle is this on? I would expect the fiesta would have 1 pre-cat o2 sensor and 1 post-cat o2 sensor. Perhaps the app you're using mislabeled the o2 sensor? On a vehicle with a V engine and a pre-cat o2 sensor on each bank, I would expect to see 4 fuel trims: bank 1 short term, bank 2 short term, bank 1 long term, bank 2 long term

I'm fairly certain "some_other_dave" has the right idea on tracking this down. Start with the known issue first, the failing O2 sensor before moving on the fuel trim concerns.